Pathogenesis studies have indicated that colonic mucosal cell proliferation in mice precedes evidence of an inflammatory response to the inciting agent. Kinetic studies indicate marked increases in mitotic activity not only at the lower 1/3, but throughout the entire width of mucosa including occasional surface epithelium. Different strains of mice demonstrate varying degrees of susceptibility indicating a genetic host variability. Induction of the lesion can be accomplished with pure cultures of a strain of bacteria tentatively classified as a variant of Citrobacter freundii. Stock cultures of Citrobacter freundii have not produced this lesion. Millipore filtrates 0.45 u of colonic mucosa are non-infective and virologic studies have not implicated a virus to date, although this has not been completely ruled out. The unique features of this proliferative disease of colonic mucosa are being further studied as to: 1) possible interaction of a virus with the bacterial isolate; 2) effects of metabolic products of this bacterial agent on inducing disease; 3) effects of agent on different animal species, 4) effects of introducing an intestinal carcinogen at low levels for possible synergism.